It started with a chance conversation between a doctor and a nurse several years ago. But that brief encounter may end up exposing what could be one of the largest Medicare frauds in U.S. history.

Dr. Alon Vainer, a medical director at dialysis clinics in Georgia, was discussing clinic procedures with one of the nurses, Daniel Barbir. The two men say they saw something they believed was very wrong: expensive medicine, and lots of it, was being tossed in the trash. And the clinic workers were being told to do it, the two men say.

"When we sat down and started talking about it and getting into details, we actually realized exactly what was going on," Vainer said.

The alleged waste was being carried out on a massive scale and, the nurse and the doctor said, they knew why almost immediately. They claim it was a way for their company, DaVita Inc., to defraud the government, overbill Medicare and Medicaid and make a fortune.